Composite-based resins have made significant contributions to restorative dentistry. Improvements in such resins have facilitated an increase in prosthodontic therapy and aesthetic dentistry. One of the principal uses of composite resins is in the manufacture and application of provisional restorations. Provisional restorations are temporary prostheses that are placed on, or in place of, one or more teeth for a limited period, from several days to several months, and in some cases even longer, and which are designed to protect the tooth or teeth, provide masticatory function, maintain proper alignment of adjacent and opposing teeth, and remain in position until a permanent restoration is facilitated. As can be readily understood, the resin that comprises a provisional restoration must have the durability, toughness and physical properties to withstand mastication of hard food while maintaining a tight fit to the underlying tooth or teeth. It must also have an aesthetic appearance that matches or improves upon the appearance of the original teeth.
Dental resins typically contain three primary ingredients: (1) an organic binder or matrix of monomers; (2) inorganic filler; and (3) a coupling agent, often incorporated into a polymerization system. The qualities of the individual components that comprise a resin, particularly the monomers used, as well as synergistic properties derived from particular combinations of components, have an enormous impact on the aesthetic quality, durability and clinical utility of the resin. The most desirable resins possess high impact strength, high elasticity, high hardness, and dimensional stability with little tendency to swell through water absorption (which lowers strength).
The organic binder of composite resins is made up of a system of mono-, di- or tri-functional monomers. The monomer system can be viewed as the backbone of the composite resin system. Suitable monomers include ethylenically unsaturated compounds such as acrylic acid and methacrylic acid esters.
Filler particles vary from one composite-based resin to another, and each type of filler has its own distinctive characteristic. The filler portion of a composite-based resin has a significant effect on the qualities of the cured composite. For example, the size of a given filler particle can affect roughness and strength. As a general rule, the higher the loading of filler, the higher the strength of the final composite-based resin.
A polymerization system is often comprised of several components. These include polymerization initiators specific for a given type of polymerization system. For example, in chemically-activated systems, benzoyl peroxide and tertiary amines serve as a source of free radicals, which create propagating sites of polymerizing reactivity in carbon-carbon double bonds of the monomers. For light-activated resin composites, a diketone photoactivator is typically used.
Polymerization systems may also contain coupling agents to bond the filler particles to the organic resin matrix via silanating agents. This serves to improve the resin's physical properties by preventing hydrolytic breakdown along the filler/matrix interface. Hydrolytic breakdown of the filler/matrix interface can crack the resin through stress transfer.
Acrylic dental resins have been in use for over fifty years and continue to be used for the fabrication of provisional restorations. These materials are typically polymethylmethacrylate (PMMA) and methyl methacrylate (MMA). With acrylic resin, the fabrication of the provisional restoration usually requires mixing a powder and liquid together to form a paste that is placed in either a premade shell (tooth form), or into a template or carrier that is placed over the tooth preparation. These materials have adequate strength, are tooth-colored and have relatively good color stability over a few weeks. They can be smoothed and polished, are easily repaired, and are relatively inexpensive. PMMA resins have a sufficiently high glass transition temperature (Tg) to allow trimming, grinding and finishing of the cured material without causing the material to soften or distort. Tg is the temperature at which a polymer goes from a hard, glass-like state to a rubber state. Since PMMA resins have a sufficiently high Tg, the heat of the grinding and finishing wheel used to trim, shape and finish the margins of the provisional restoration does not distort the resin. The material grinds and powders providing crisp and accurate margins. The accurate margins provide a good adaptation to the tooth preparation and gingival tissue.
However, repeated mixing of powder and liquid exposes the dentist and/or dental assistant to monomers that may be cytotoxic to users under certain conditions. Allergies to PMMA monomers are well documented, and these allergic reactions can be quite severe. PMMA products also have strong and objectionable odors. Furthermore, precise dispensing of PMMA products is difficult and mixing ratios generally vary according to user experience and desires. Because of these variations in proportions and mixing skills, physical properties can vary, irritation to oral mucosa can be exacerbated and there can be an increase in the exothermic reaction, heat gain, which can produce negative effects on the dental pulp if not mitigated with specific, seldom used additional techniques and materials.
In addition, PMMA materials are well known for shrinkage during polymerization leading to poorly fitting restorations. Following the initial insertion of the carrier matrix (template) filled with PMMA, the carrier matrix with partially polymerized PMMA restoration is removed from the mouth before the final set, and complete polymerization and hardening continues outside the mouth. The torque and manipulation applied when removing the carrier matrix can distort the provisional restoration, adding another contributing factor to the poor fit of PMMA provisional restorations.
Additional shrinkage continues to occur over time in the mouth due to the constant exposure to moisture in the oral environment. The changing dimensions of the PMMA restoration change the way the prostheses fit, causing them to come loose, and a PMMA restoration may need to be relined in cases deemed long term. The effect of shrinkage at the margins increases the micro-leakage of bacteria and may allow re-infection of the tooth prior to the placement of the final restoration.
Due to the deficiencies of the aforementioned PMMA acrylic resins, Bis-acrylic resins have become a popular material for provisional restorations. Bis-acrylic composites have less exotherm, are easier to mix precisely due to automix dispensing systems, are more polishable, more color stable, have better physical properties and shrink less than PMMA resins.
However, Bis-acrylics have a number of prominent disadvantages, including high cost, low Tg, difficulty in making repairs, brittleness, frequent breaking at pontic areas, suitability for single-unit provisionals only, and frequent debondings requiring re-cementation. Unique to bis-acrylic resins is a low glass transition temperature resulting in gumming up of finishing instruments and the routine softening and loss of margins at the tooth preparation interface. Bis-acrylics soften from the heat produced by the grinding and finishing instruments, and this distorts the margins. The bis-acrylic must then be relined and the margins must be re-established with flowable composites. This aspect is time consuming and may create resin compatibility issues. Marginal integrity is critical to proper fit and the health of the gingival tissue.
Bis-acrylics have adequate flexural and compressive strengths, but are quite brittle, lacking the desired toughness and deflection at break. Also, they do not have a sufficient “memory effect” (also referred to as “temporary flexibility effect”) to maintain an optimal marginal fit.
Increasing toughness and flexibility without sacrificing other properties is a great advantage in dental restorative applications. Various approaches have been tried to address these issues. In 1994, May et al. (U.S. Pat. No. 5,376,691) disclosed adding non-polymerizable additives (such as polyethylene glycol) or plasticizers (such as esters of phthalates), but these approaches yield polymers with inferior mechanical properties such as brittleness, incomplete curing, phase separation and leaching of non-polymerized additives. Recently, Orlowski et al. disclosed the use of polybutene in acrylate resins to increase the flexibility and decrease the brittleness of dental prosthetic materials. However, polybutenes are non-polymerizable, low Tg materials and may create micro-nonhomogenieties in the resin; polybutenes may leach out of the cured resin matrix, decreasing the durability of the dental composite; and the addition of polybutenes will not eliminate the gumming of the crowns and bridge material on polishing.
Urethane dimethacrylate resins have been used to fabricate dental prostheses. Urethane dimethacrylate resins have excellent flexural characteristics but lack flexibility. Also, high amounts of low molecular weight diurethane dimethacrylate in the resin formulation increases the exothermic heat and increases polymerization shrinkage due to the high concentration of double bonds.